The present invention relates to an electrophotographic recording material composed of an electrically-conducting coating support which is suitable, in particular, for the manufacture of printing forms or printed circuits, and a photoconductive insulating coating.
In order to manufacture printing forms, it is known to use photoconductive, monomeric organic compounds, or photoconductive resins composed of condensation products of formaldehyde with various carbocyclic compounds (U.S. Pat. No. 3,842,038) wherein the photosensitivity of these materials generally is in the region between 350 and 430 nm.
In order to bring about the bathochromic shifting of the spectral sensitivity ranges of electrophotographic copying materials, down to approximately 800 nm, it is known to employ sensitizing dyes which are homogeneously dissolved in the photoconductive insulating coating, such as polymethine dyes, triphenylmethane dyes, phthalein dyes, etc. (U.S. Pat. No. 3,189,447; British Pat. No. 944,126; U.S. Pat. No. 4,063,948; U.S. Pat. No. 4,252,880).
The extension of the spectral photosensitivity can also be brought about through the addition of chemical activators, for example, electron-acceptors, according to U.S. Pat. No. 3,287,120, as has been described in German Pat. No. 2,726,116 for a condensation product of formaldehyde with 3-bromopyrene.
The electrophotographic recording material possesses a coating support which is composed of metal, or which has been metallized, and which is suitable for printing purposes. As a rule, the photoconductive insulating coating is composed of a mixture of the photoconductor, a homogeneously dissolved sensitizing dye, and/or an acceptor-compound, and a binder possessing a high molecular weight and containing groups which confer solubility in alkaline media. The plate is electrostatically charged, exposed, and developed with an electrophotographic developer, of the dry type, or of the liquid type. The toner-image obtained is fixed, for example, by heating to 100.degree. C. to approximately 250.degree. C., after which the insulating layer is treated with an aqueous, or alcoholic, alkaline solution, whereby the non-image areas are dissolved away, and a lithographic printing form is produced.
The known recording materials have the disadvantage that their photosensitivity does not yet meet the highest requirements. For this reason, improved photosensitivity of the copying materials is becoming increasingly important in the commercial production of printing forms by automatic techniques. Moreover, it is disadvantageous that these photoconductive coatings possess enhanced conductive properties in the dark, caused by the homogeneously dissolved sensitizing additives, which are frequently present as salts. A certain sensitivity to pre-exposure is also a disadvantage, which necessitates that these materials be stored and handled in the dark.
It is less easy to prepare lithographic printing forms according to German Pat. No. 2,726,116 and German Offenlegungsschrift No. 2,755,851, due to the processes for manufacturing their components from a photoconductive resin of constant quality and acceptor compounds.
Electrophotographic recording materials with photoconductive double layers are also known (British Pat. No. 1,416,603 and U.S. Pat. No. 4,028,102), in which the dyes are present exclusively as constituents of a coating which serves to produce charge carriers. It has, however, been found that the double process step of vapor-depositing the dye coating and the application of a charge-transporting coating is disadvantageous for the manufacture of printing forms and printed circuits, particularly with respect to the later operation of dissolving away the non-image areas, because only incomplete detachment of the dye coating occurs.